The present invention relates to attenuators for radio frequency (RF) energy and, more particularly, to a thin Salisbury screen using closely spaced frequency selective surfaces.
Modern communications technology often requires radio frequency absorbing surfaces to achieve isolation between antennas and sometimes adjoining structures on host platforms. Applications such as providing isolation fences around antennas are typical for these absorbing structures. Traditional absorptive structures such as carbon-based surfaces generally need to be on the order of one wavelength thick to provide the required absorptive performance. Magnetic-based absorbers may be thinner but are generally much heavier, because of their dependence upon iron loading. This makes magnetic absorbers unsuitable for use in weight-conscious applications, in applications where the absorptive structures must withstand either atmospheric exposure or exposure to other corrosive materials. There has been a need to develop thin, lightweight, RF-absorptive structures which are capable of broadband absorptive performance.
The Salisbury screen is one well-known approach to achieving high degrees of RF-absorption over a narrow frequency band. U.S. Pat. No. 2,599,944 for ABSORBENT BODY FOR ELECTROMAGNETIC WAVES, issued to Winfield W. Salisbury, describes such a structure. SALISBURY teaches a composite structure which may be placed over essentially any surface to render that surface electromagnetically non-reflective. SALISBURY uses a graphite-coated canvas, spaced apart from a metal back surface (i.e., a ground plane) by wood blocks. The spacing is dependent U on the frequency to be absorbed, generally approximately xcex/4. Circuit and transmission line theories may be used to show that the ground plane, which is a short circuit (≈0xcexa9 impedance), is transformed to an open circuit (≈∞xcexa9 impedance) at xcex/4 distance from the ground plane. By placing the resistive sheet at xcex/4 location, a 377xcexa9 impedance is placed in parallel with the reflected open circuit. This results in a structure in which an incident plane RF wave, which has a 377xcexa9 impedance in free space, is matched to the 377xcexa9 load sheet which then totally absorbs the incident wave""s energy.
This effect occurs only at a single frequency. For this reason, Salisbury screens in their pure form have found little usage in practical, broadband RF-absorptive structures. In a typical application, an RF-absorptive structure might be required to absorb an incident, radar signal. While the Salisbury screen can be highly effective at a single frequency, the ease with which the radar system may be tuned to a different operating frequency renders the Salisbury screen essentially useless.
A broadband structure of a similar construction, however, could be quite useful. U.S. Pat. No. 5,1627,541 for INTERFERENCE TYPE: RADIATION ATTENUATOR, issued to Donald D. Haley, et al. teaches on such structure. HALEY, et. al. expands the concept of the Salisbury screen by placing a xe2x80x9cspaceclothxe2x80x9d in front of a plurality of reflective layers, each of the reflective layers being tuned to reflect a narrow range of frequencies. By properly placing the layers, the overall absorption of the structure may be increased. Each of the reflective layers still must be spaced xcex/4 from the spacecloth. Each reflective layer must also be essentially transparent to other frequencies. Frequency selective surfaces (FSS), well known to those skilled in the RF arts, may be used to construct the HALEY, et al. structure. Still, a structure built in accordance with the teachings of HALEY, et al., capable of true wideband absorption, is unwieldy (i.e., thick) and expensive and, therefore, impractical for most modern applications.
The inventive wideband absorptive structure, however, overcomes many of the problems of the HALEY, et al. structure. The structure of the instant invention utilizes a spacecloth with a 377 ohm bulk impedance placed in front of a plurality of frequency selective surfaces. The spacings between the spacecloth and the individual reflective layers are not the traditional xcex/4, but rather much closer spacings are utilized. The inventive structure, unlike that of HALEY, et al, utilizes the mutual coupling between the closely spaced FSS layers.
In a traditional Salisbury screen structure (e.g., that of HALEY, et al.) the amount of absorption decreases rapidly as either the frequency of the impinging signal deviates from the frequency to which one of the FSS layers is xe2x80x9ctunedxe2x80x9d or as the angle of incidence of the impinging wave deviates from normal impingement. The inventive absorptive structure, on the other hand, is responsive to RF energy at a much greater degree of deviation from normal incidence.
It is therefore an object of the invention to provide a broadband absorptive structure having a thickness less than xcex/4.
It is a further object of the invention to provide a broadband absorptive structure utilizing a plurality of FSS reflective layers spaced closely together.
It is an additional object of the invention to provide a broadband absorptive structure wherein the closely-spaced FSS reflective layers mutually interact to reflect a coherent signal (0 degrees phase) at a reference plane that is less than xcex/4 distance from the first FSS layer.
In accordance with the present invention, there is provided a broadband, RF-absorptive structure based on a Salisbury screen. A plurality of closely-spaced FSS reflective layers interacts with a ground plane and each other to reflect a coherent return signal over a broad bandwidth to a spacecloth front layer. The frequency response of the absorptive structure is relatively flat across an octave (i.e., a 2:1 frequency ratio) bandwidth. The overall thickness of the inventive structure is lese than xcex/4 because of the interactions of the FSS layers and the ground plane.